Functional parts of rp and a devices and conventional symbols used in them

Fig 3.5. NC contact


A combination of contacts
A relay may have a combination of the above contacts. Look at the following illustration

Fig 3.6. A combination of contacts

In this case, there is a 3rd terminal called “COMMON”. The NO and NC contacts are referred to the COMMON terminal. Between the NC and the NO contact, there is no contact at any time!
The following figure shows how this pair operates:

Fig 3.7. Operation of combination of contacts

OK, we have the NORMAL open and NORMAL closed contact. But which state is considered as NORMAL? Going one step closer to the relay operation, we find the spring.
This spring defines the NORMAL position of the COMMON contact. If you see the above 3 animations, you will notice that one time an F force is applied to the COMMON terminal, and the other time there is no force applied. Well, this is actually wrong.
There is indeed another force that pulls the contact towards UP and this force is applied ALL the time. This force comes from the spring. Look the following image:

Fig 3.8. Spring of contacts

Now you can see what is pulling the COMMON terminal UP all the time. So the spring defines what is the NORMAL state, and thus defines which contact is the NORMAL OPEN and which the NORMAL CLOSED. In other words, the NORMAL state is defined as the state that there is NO other force applied to the COMMON terminal except the one from the spring.